How to make a cross-platform service?

Question

Mobile development

Hello! He gave birth to a promising idea (as it seemed to me), partly related to the existing offline business. I am 200% sure that this will work and will be in great demand both in large cities and not so much. Of course, there will not be SUCH a mass character, as, for example, with Avito - the potential audience is somewhat smaller, but at least a million users will be happy with the opportunities that the service should provide. There are no complete analogues, the idea is quite original. Solves the pain of hundreds of organizations like mine, tens of thousands of low-income people, and provides new opportunities and a source of customers for a huge number of organizations. Therefore, I am in no hurry to disclose the details. It seems that I got flustered, daydreamed and forgot what I wanted to ask, sorry .. Oh yes. Let's not go into all sorts of little things like millions for promotion - I don't know what to do with it and where to get the money. But in order to promote something, you need to create something, you should probably start with this.
Given the potential number of users, the service may well turn out to be highly loaded. The audience coverage should be as large as possible, therefore - the service should have a web version and applications for iOS, Android and WP - in short, a gentleman's kit. Also, the service should have a "special place" like a personal account for an organization, where entrepreneurs can register their organizations and provide users with information about themselves. It is desirable to be able to integrate with existing accounting systems with popular accounting systems, like 1c (but this is probably difficult, right?). In addition, the possibility of advertising their organizations within the service should be implemented (no, this is not an analogue of 2gis), and various paid functions (like highlighting an ad on Avito). And to everything else, special algorithms should be developed, something like a trust level for users and organizations that automatically calculate how much you can trust a particular user / organization. As we all know, a website, a mobile application, and so on, is just the tip of the iceberg, a beautiful and convenient shell for the “something” that is behind it and allows it all to work.
My question is just what is this "something" - the back-end, and where to start building the core of this entire system. About money, that they need a lot - in principle, I myself guess, the amount of work here seems to me simply colossal. Can someone "on the fingers" decompose at least roughly how such large services are developed in stages? So that at least some real idea can be obtained. Because something tells you that you can start with something small, with some kind of mini version, but now your head is just a mess of all these ideas and fears. I will be very grateful for help with parsing "on the shelves." Thank you!

Answer 1

with such a wall of text and a stream of consciousness, just contact the office to:
1) create and elaborate the terms of reference
2) contact another or the same office to complete the created terms of reference.
With such Napoleonic plans, you will cut it all in one snout to a carrot conspiracy, or someone else will come up with your idea and implement it faster than you, since it solves a bunch of problems for a bunch of people.

Answer 2

From a technical point of view, this is an ordinary task.
The backend in this case is a certain server with specially written software (or a group of servers) on the Internet, to which all your mobile applications access, as well as the frontend of your website.
At a minimum, this is an API + database. And in the vast majority of cases, this is an API + the so-called "business logic" + a database.
PS:
It will cost you at least the cost of the car.
new. Perhaps domestic production, but new.
And perhaps not domestic.